1. Field of the Invention
The present invention is directed to minimizing CO2 emissions from Gas-to-Liquids (GTL) facilities. In particular, the present invention is directed to reducing CO2 emissions from GTL facilities such as, for example, Fischer-Tropsch facilities, by using hydrogen as a fuel used in the GTL facilities.
2. Description of the Related Art
The conversion of natural gas assets into more valuable chemicals, including combustible liquid fuels, is desired to more effectively utilize these natural gas assets. The conversion of natural gas to more valuable chemical products generally involves syngas generation. Syngas generation involves converting natural gas, which is mostly methane, to synthesis gas or syngas, which is a mixture of carbon monoxide and hydrogen. Syngas can be used as a feedstock for producing a wide range of chemicals, including combustible liquid fuels, methanol, acetic acid, dimethyl ether, oxo alcohols, and isocyanates.
There are two main approaches to convert remote natural gas assets into conventional transportation fuels and lubricants using syngas. Natural gas may be converted into syngas followed by a Fischer-Tropsch process, or natural gas may be converted into syngas followed by methanol synthesis, which is followed by a methanol to gas process (MTG) to convert methanol into highly aromatic gasoline. The syngas generation is the most costly step of these processes. A critical feature of these processes is producing syngas with a desired H2/CO ratio to optimize formation of the desired products and to avoid problems in the syngas formation step.
Syngas can be generated from three major chemical reactions. The first involves steam reforming of methane. The ratio of hydrogen to carbon monoxide, which is formed from this process, is typically approximately 3.0. A second process for syngas generation involves dry reforming of methane or the reaction between carbon dioxide and methane. An attractive feature of this method is that carbon dioxide is converted into syngas; however, this method has problems with rapid carbon deposition. The carbon deposition or coke forming reaction is a separate reaction from the one that generates the syngas and occurs subsequent to the syngas formation reactor. However, the reaction of methane in dry reforming is slow enough that long residence times are required for high conversion rates and these long residence times lead to coke formation. The ratio of hydrogen to carbon monoxide, which is formed from this process, is typically approximately 1.0. A third process for syngas generation involves partial oxidation of methane using oxygen. The ratio of hydrogen to carbon monoxide, which is formed from this process, is typically approximately 2.0. However, in commercial practice, some amount of steam is typically added to a partial oxidation reformer in order to control carbon formation and the addition of steam tends to increase the H2/CO ratio above 2.0.
It is possible to produce syngas with a H2/CO ratio that is above the ratio ideally desired for the process in which the syngas is to be used, and then to remove excess hydrogen to adjust the ratio to the desired value. However, the H2 removal process employs expensive H2 separation systems that tend to foul and decline in performance with use.
The Fischer-Tropsch and MTG processes both have advantages and disadvantages. For instance, the Fischer-Tropsch process has the advantage of forming products that are highly paraffinic. Highly paraffinic products are desirable because they exhibit excellent combustion and lubricating properties. Unfortunately, a disadvantage of the Fischer-Tropsch process is that the Fischer-Tropsch process emits relatively large amounts of CO2 during the conversion of natural gas assets into saleable products. An advantage of the MTG process is that the MTG process produces highly aromatic gasoline and LPG fractions (e.g., propane and butane). However, while highly aromatic gasoline produced by the MTG process is generally suitable for use in conventional gasoline engines, highly aromatic MTG gasoline may be prone to form durene and other polymethyl aromatics having low crystallization temperatures that form solids upon standing. In addition, the MTG process is more expensive than the Fischer-Tropsch process and the products produced by the MTG process cannot be used for lubricants, diesel engine fuels or jet turbine fuels. Furthermore, like the Fischer-Tropsch process, the MTG process also generates CO2.
Hydrogen recovered during petrochemical processing has been used for various purposes. For example, U.S. Pat. Nos. 6,043,288 and 6,103,773, and 6,147,126 to Exxon describe recovering hydrogen from syngas for uses including hydrotreating and catalyst regeneration, while CO rich offgas is used for fuel.
In another example, BP has disclosed using a steam reformer followed by a membrane separator to recover excess hydrogen which is used as a fuel gas in the steam reformer. (“Alchemy in Alaska,” Frontiers, December 2002, pages 14-20).
In addition, WO 00/69990 and WO 00/69989 describe producing hydrogen from light products produced from hydrocracking for use in various operations, including hydrocracking. The feedstock used in the disclosed processes can be a Fischer-Tropsch feedstock. However, the methods of hydrogen production described in WO '990 beginning at page 9, line 30 and in WO '989 at page 12, lines 11-17 include partial oxidation, steam-methane reformation and catalytic dehydrogenation.
Finally, EP 635555A describes using naphtha reformation to produce hydrogen used for upstream hydrotreating. EP '555 refers to the refining of petroleum products.
There remains a need for efficient processes to convert a methane-containing feedstock into hydrocarbonaceous products in a GTL facility and to minimize CO2 emissions generated by such GTL processes.